Same problem for me. Operating lever not locking down. And same solution! After removing the base plate on the kettle one of the three screws holding the complicated mechanism to the kettle itself was completely loose. After a quick screw up it worked perfectly again. Jim
I fixed this problem by removing the base (you can remove the screws with a small screwdriver and a bit of care) and then tightening the three screws that hold the switch and thermostat in place.
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